An actuator of this type is, for example, known from the international patent application WO 2009/119450. Such an actuator makes it possible to produce an electromagnetic device which can be used when positioned against a spring or with a reciprocating movement in an oscillating and vibrating mechanical system. Various devices are known in the state of the art, which aim at solving the general problem of the force linearity they exert as a function of the current supply they receive, and at developing force levels which are sufficient for numerous applications. The documents EP0592647 and EP1005714, which belong to the Applicant, refer to electromagnetic actuators developing an effort proportional to the current through the mutual action of the magnetic fields created by the permanent magnets they contain and the feed electric coil.
Such electromagnetic devices are particularly interesting when they are used in oscillating systems, when combined with resilient return members, where the proportional characteristic thereof and the strong force density thereof make it possible to check heavy loads, whether of a dry or viscous nature. Such devices comprise two identical movable portions moving in phase inversion, cancelling the total movement amplitude, and thus guaranteeing an optimum balance of the actuator. They are more particularly used for vibrating contra-rotating mechanical members such as, for instance, the blades of an electric razor.
This solution has a drawback in that the force density enabling a satisfactory operation requires a structure with large dimensions. When the geometric dimensions are reduced into a compact configuration, and more particularly the thickness of the structure, this results in situations wherein the force density is insufficient. A first problem that the invention aims at remedying is that of the reduction in overall dimensions, in order to enable the production of very compact actuators, without affecting the performances thereof.
Such devices of the prior art may be inefficient when the need for a significant miniaturization of the actuator exists. The devices of patents EP0592647 and EP1005714 thus develop too low force factors (force per ampere-turn), which makes the utilization thereof non viable with small dimensions, since heating is too important. As a matter of fact, the reduction in overall dimensions leads to a reduction in the volume reserved for the electric coil. A more important electric power will thus be necessary for producing the same force. Such more important electric power will result in too high current consumption, thus limiting the service life, with an equivalent battery, when the system is aboard, and will result in a higher Joule dissipation. This may lead to too high a heating of the device causing some inconvenience for the user in the case of a portable application such as an electric razor, or to the destruction of the actuator.
The international patent application WO 2009/119450 mentioned above takes up the utilization of such electromagnetic structures by recommending the elimination of the ferromagnetic yoke, with a view to favouring construction simplicity. In the structure known from such application WO 2009/119450, the reduction in the movable ferromagnetic portion certainly makes the construction easier with fewer parts, but it also requires a much larger magnet volume, which finally results in higher cost, larger movable mass and global dimensions. The second technical issue that the invention tries to remedy consists in reducing the required magnet volume, which turned out to be possible only when implementing a solution against the teachings of the above-mentioned prior art.
The present invention aims at remedying the drawbacks of the prior art by producing an electromagnetic actuating device having a higher force density than the devices of the prior art. The invention more particularly enables the utilization of an efficient actuator in a reduced volume, relative to the devices of the prior art. For this purpose, the present invention, which is moreover complying with the generic definition given in the preamble above, is mainly characterized in that the magnetic portion further comprises a yoke contiguous to the magnetized areas and closing off the magnetic fluxes passing through the magnetized areas, in that the course of travel C of the magnetic portion is less than or equal to the width LP of each pole of the stator unit, and in that the thickness of the magnetized areas perpendicular to the direction of movement is less than the course of travel C of the magnetic portion. In the meaning used herein, the word “course” means the maximum distance between two extreme positions taken by the magnetic portion relative to the stator unit.
In the state of the art, U.S. patent application publication No. 2002/0055695 is known, which discloses an actuator having a stator comprising a first set of poles and a second set of poles distributed on each side of an electric coil positioned about a central element. The number of poles of the stator on each side of the coil is the same. When the coil is powered, there are as many South poles as North poles in the stator.
In this solution of the prior art, the electric coil does not surround a pole but a central portion separating the North poles from the South poles. The stator disclosed requires a particular design, as illustrated in FIGS. 3a and 3b, with an alternating set of packaged sheets which make up together an elementary pattern. This entails a high production cost of the stator. The movable element composed of magnets, moves in the air gap existing between two opposite poles of the stator and has no yoke closing off the magnetic flux. FIGS. 9 and 10 of this document of the prior art show twice as many poles of the stator as magnetized poles (with each “tooth” of the stator representing a pole).
Patent DE102005044176 is also known in the prior art, which discloses an actuator with a two-pole stator. In this solution, each termination of the two poles of the core faces a magnet transition, i.e. an area separating a North-permed pole and a South-permed pole. When the stator moves relative to the coiled core, each pole termination faces a North pole (if the stator moves to the right) or a South pole (if the stator moves to the right).
Thus, in theory, the flux produced by the magnets and passing through the coils is null, whatever the position. This results in an actuator which cannot produce an effort with a current. In order to make an effort with a current, when the stator moves relative to the magnetized portions, each pole must face a different magnet polarity in order to provide a variation in the magnetic flux in the coil, and this flux variation will create the electromagnetic force.
Patent EP1005714 is also known in the prior art, which discloses a magnetic actuator the basic pattern of which is identical and which is composed of a two-pole stator and one or two movable portion(s) having 3 alternate magnets. The invention provides for the modification of the magnetic structure of the actuator disclosed in patent EP1005714 by increasing the force factor thereof in the same volume, without modifying the intrinsic characteristics thereof which are the coil volume and permeance. It is thus possible to produce, according to the invention, an actuator the volume of which is substantially less than the volume which would be required by an actuator produced according to the teachings of the patent EP1005714 to develop a force having the same intensity.
Adding a mild ferromagnetic yoke makes it possible, contrary to the teachings of the prior art WO 2009/119450, to substantially increase the performances of the actuator by efficiently closing off again the magnetic flux and by limiting the loss of magnetic potential. Limiting the course of travel C to a value less than or equal to the width of a pole belonging to the stator unit makes it possible to keep an optimum ratio of force to magnet mass and to keep a quasi-linear force characteristic along the course of travel. Limiting the thickness of the magnetized areas to a value less than the course of travel C of the magnetic portion, which is, for instance, made possible by using high energy magnets of the NdFeB or SmCo types, makes it possible to obtain a force density per actuator volume and per magnet mass which is higher than those of the structures of the prior art.
In the actuator of the invention, N is for instance 3, and such actuator preferably includes a single coil surrounding the central pole of the stator unit. The magnetized areas advantageously comprise at least two inner magnetized areas having, along the direction of movement, a length LA2 equal to the distance between the respective median planes of two consecutive stator poles, with such median planes extending perpendicularly to the direction of movement. Besides, the magnetized areas comprise at least two end magnetized areas preferably having, along the direction of movement, a length LA1 between LP and 1.2×LP.
According to a first possible embodiment, the magnetized areas are formed by independent and contiguous magnets fixed on the yoke. According to another possible embodiment, the magnetized areas are formed by a single magnet, having magnetized areas alternately magnetized in opposite directions and transitions having a length less than 0.2×LP. As a matter of fact, when the magnetic transition between two magnetized areas is above such factor, the loss of force becomes significant for the same magnet mass.
According to an alternative solution, the actuator comprises two movable magnetic portions moving in opposite directions, which makes it possible to make up for the movement amplitude of the two portions. According to another alternative solution, the actuator comprises three movable magnetic portions moving in alternate directions, which makes it possible to make up for the movement amplitude and the kinetic movement of the three portions.
The stator unit is advantageously connected to each magnetic portion by suspension blades. According to a particular alternative solution, the stator unit is connected to each magnetic portion by resilient return elements so designed as to return such magnetic portion to the middle of the course of travel thereof. The coil is advantageously so designed as to be powered by alternate current and to produce a vibratory movement of the magnetic portion relative to the stator unit.
According to a possible embodiment, the surfaces of the stator poles are, for instance, inscribed within a cylindrical enclosure with the magnetic portion(s) having a semi-tubular shape cooperating with the cylindrical enclosure. In addition, the yoke has chamfers at the ends thereof, along the direction of movement. It may also be advantageous to provide for each pole of the stator unit to have a head having a width LP and a body having a width less than LP.
In some applications, the actuator of the invention can further comprise a magnetic sensor and an electronic control circuit, with the magnetic sensor being positioned opposite the magnetic portion and so designed as to provide an output signal representative of the absolute position of such magnetic portion, and the electronic control circuit receiving the output signal from the magnetic sensor and being so designed as to control the position of the magnetic portion in a position determined by a set value powered to such electronic control circuit. The invention further relates to the use of an actuator such as described here-above for an electric razor comprising a head provided with vibrating knives, characterized in that each one of the knives is kinematically linked with the magnetic portion of such an actuator.
Adding two end magnets to the magnets of the magnetic device as disclosed in document EP1005714 makes it possible to create a force factor higher by a 4/3 factor, i.e. a 33% increase for the same current, the same coil, the same electric power and the same volume of the actuator. As a matter of fact, when N=3 and when one and only one coil is positioned on the central pole of the stator unit, the value of the theoretical force F produced by the actuator, equal to the product of the force factor by the number of ampere-turn of the coil, executed for instance using an equivalent electric diagram, is given by the following relation:
  F  =                    8        ⁢                  B          r                ⁢        LZ                    3        ⁢        E              ⁢    ni  
where Br is the magnet remanence, L the thickness thereof in the direction of the magnetization thereof, Z the depth thereof, E the air gap including the magnet, and ni the number of ampere-turn of the coil.
In the actuators of the state of the art, having a yoke, as mentioned above and including a single coil, as disclosed in documents EP1005714 and EP0592647, the factor applied to the quantity
                    B        r            ⁢      LZ        E    ⁢  niis not 8/3 but 2.